This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present invention. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present invention. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
Mooring line failure of an offshore permanent floating structure can result in financial consequences and physical damage, particularly in oil/gas fields. Significant financial damages due to a mooring system failure may result from high cost of repair or replacement of the damaged mooring line, production shut-down, and/or long lead procurement of the new mooring component. Mooring failure records show that production semis have a failure every 9 years and every 8.8 years for FPSOs (Floating, Production, Storage, and Offloading). See M. G. Brown et al., “Floating Production Mooring Integrity JIP—Key Findings”, OTC 17499 (2005); “Analysis of Accident Statistics for Floating Monohull and Fixed Installations”, HSE Research Report 047 (2003). Challenges in repair or replacement of broken mooring lines include spare line costs, project dependency of sizes and configurations of mooring, maintenance challenges, and long lead time for procurement of new mooring line components.
A conventional permanent mooring line system is depicted in FIG. 1. As vessel 101 floats in the water 103, it is held in place by the combination of mooring line 105 and anchor 107. Mooring line 105 is fixed to vessel 101 and anchor 107. Anchors 107 are held in place by being driven into the seabed 109. As a load is placed upon vessel 101, the tension within the mooring lines 105 increases. Once the tension passes the threshold the mooring line can withstand, mooring line 105 will break thereby creating an unsafe condition for vessel 101 and the surrounding equipment, which may include, but is not limited to, risers/umbilicals, subsea pipeline and equipment, and other oil/gas production equipment.
In the case of a mooring line bundle, when a plurality of mooring lines is fixed to the vessel, one of the lines within the bundle is typically subjected to the greatest load. When the tension exceeds the threshold for that line, the mooring line will fail (i.e., break). Naturally, the loss of one mooring line causes an increase in tension within the other mooring lines of that bundle. Unless the load on the vessel is reduced, there is a significant likelihood that the loss of one mooring line will result in the failure of the other lines within the bundle, inevitably leading to undesirable consequences: financial, safety, or otherwise.
Presently, there is no mechanism to prevent failure of a mooring line 105 due to the vessel 101 being driven by the extreme load such as, but not limited to, squalls, icebergs, and hurricane. Although permanent mooring systems of offshore floating structures are designed and installed with design margin, there has been a growing concern about the safety factor of the mooring system which may not be able to meet industry standard requirements due to: (a) metocean criteria update, (b) expansion of the existing project, and (c) uncertainty of “actual” extreme loads (e.g., squall, icebergs, hurricane, etc.).
Thus, there is a need for improvement in this field.